A magnetic recording medium is widely used for recording electromagnetic signals, and there are various shapes of a magnetic recording medium such as a tape, a disk or a card according to usage. A magnetic recording medium tape is used for audio recording, video recording and for computers. Hereinafter, a magnetic recording medium tape is referred to as a "magnetic tape".
A magnetic tape is basically composed of a nonmagnetic support and a magnetic layer containing ferromagnetic particles provided thereon. A magnetic tape further provided with a backing layer (back coat layer) on the opposite surface has been developed to improve running efficiency and is now used.
The backing layer using polyester polyurethane or polyether polyurethane as a binder and containing nonmagnetic particles dispersed in the binder has conventionally been known.
Polyester polyurethane has advantages such as a high tear strength, high solubility in a selected solvent, or the like, but has the disadvantage of extremely poor hydrolysis resistance. Although polyether polyurethane is excellent in hydrolysis resistance, it is much poorer than polyester polyurethane in mechanical strength and magnetic characteristics necessary for a magnetic tape when polyether polyurethane is used as a binder.
Accordingly, a magnetic recording medium using polyester polyurethane or polyether polyurethane as a binder for a backing layer has the defects that electromagnetic characteristics and running durability are greatly deteriorated at a high temperature and a high humidity.
One purpose of a backing layer is to improve running efficiency of a magnetic recording medium, and it is very important to develop a backing layer with good characteristics at a high temperature and a high humidity.